That is a Markdown blockquote. On the desktop site you can simply select the text you copied, then press the blockquote icon on the toolbar. On mobile or if the toolbar doesn't work you can do that by starting the line with a
> character. For longer texts all lines (including empty ones) should be marked like this.
> First quote
> Second paragraph
Generally if you encounter a feature of the posts you can always click the edit button to see the source code of the post where you can see how stuff, like blockquotes were done. There is also a documentation on all markdown features at https://expatriates.stackexchange.com/help/formatting